The fibre-based packaging material of product packages, such as packing paper or board, is usually provided with a polymeric coating that makes the package tight and by means of which the package can be closed by heat sealing. Similar polymer-coated paper or board is also used for disposable tableware, such as heat-sealed drinking cups. Polyolefins, such as low-density polyethylene (LDPE), are widely used for coating due to their good heat-sealability. A disadvantage of the usual polyolefins is, however, that they are not biodegradable.
Polylactide (PLA), which has reasonably good moisture and gas barrier properties that are sufficient to many applications, has been used as the coating polymer of biodegradable packaging material; however, its use involves a number of problems. Polylactide as such is stiff and fragile, requiring a high extrusion temperature and a fairly large layer thickness to stick to the fibre substrate of the packaging material. Because of the high temperature, polylactide runs the risk of deterioration, and in extrusion, the edges of a molten web tend to tear and pin holes easily remain in the extruded layer.
To solve the extrusion problems EP-1094944 B1 discloses an inner adhesion layer, which is co-extruded together with an outer polylactide layer and which consists of a biodegradable polymer, examples of which, according to the specification, include some commercial copolyesters, cellulose esters, and polyester amides.
Another problem with the use of polylactide in the outer coating layer of the packaging material is its fairly high melting point, and the resulting poor heat sealing ability. As an improvement to this US-2002/0065345 A1 discloses a biodegradable aliphatic polyester that is blended with polylactide, its portion in the mixture being at least 9%, and a tackifier, its portion in the mixture being at least 1%. As suitable aliphatic polyesters, the publication mentions polycaprolactone (PLC) and polybutylene succinate adipate (PBSA). According to the reference the mixture can be extruded into a film, which can be axially or biaxially stretched and which can be attached to the fibre substrate by lamination.
US 2005/0192410 A1 describes polylactide films and coatings, wherein the processibility of polylactide is improved by blending with it 10-40 weight-% of polycaprolactone and 5-10 weight-% of mineral particles. According to the reference the mixture can be used in extrusion coating, but there is no mention of its adhesion to the fibre substrate or of its heat-sealability. Instead, the reference describes intermediate layers between the substrate and the PLA-based coating layer or the top layers that come on top of the PLA layer; cf. section [0039] in the specification.
US 2007/0259195 A1 describes polylactide-based films, which contain, blended therewith, 0.1-10 weight-% of biodegradable polymeric additive, the purpose of which is to increase the crystallinity of the polylactide, improving its heat resistance. As examples of such additives the reference presents FEPOL 2040 marketed by Far Eastern Textile, Taiwan, and Ecoflex marketed by BASF, both of which comprise polybutylene adipate terephthalate (PBAT). According to the reference the mixtures can be extruded onto the fibre substrate in a conventional manner, but there is no mention of the adhesion of the mixture to the substrate or of the heat-sealability of the coating thus obtained. In the reference the intended improved heat resistance of PLA does not, however, suggest improved heat-sealability but rather its weakening.